This invention is directed to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing of a semiconductor device which is characterized by a process wherein a p-type impurity layer is diffused by implantation of beryllium (Be) ions into a germanium (Ge) substrate.
In the art to which this invention pertains, it is generally known and practiced to introduce p-type dopants, such as boron (B), indium (In) or zinc (Zn), into a semiconductor wafer which already has a background concentration of n-type impurities. In the case of diffusion of boron, for example, its diffusion coefficient is relatively small, so that a subsequent heat treatment at a high temperature for a long period of time is necessary in order to obtain a deep lying diffusion. The heat treatment of the kind just described can bring about adverse effects in the semiconductor water. One such effect is a deficiency of crystal structure at the interface of an insulating film (such as a silicon dioxide film partially formed on the surface of a germanium substrate for masking the impurity) and the bulk of the germanium. There can also be thermal deformation of the whole germanium wafer. Both results are due to the difference between the thermal expansion coefficients of the insulating film and the bulk.
In the manufacture of an avalanche photodiode (APD), it is also generally known and practiced to form guard rings to improve breakdown voltage characteristics. In other words, it is required to obtain a substantial diffusion depth and a graded p-n junction to maintain sufficient withstand voltage characteristics at the p-n junction where light is irradiated. As pointed out above, boron can only be diffused shallow and the resulting p-n junction is more like an abrupt step junction. And thus, efforts have been made to find a p-type impurity material which can be diffused sufficiently deep into a germanium substrate to produce a graded junction and which exhibits high withstand voltage characteristics.
Historically silicon was used from the beginning to make a photodiode in the field of light communication systems using optical fibers. Originally, a 0.8 .mu.m band was enough for the purpose of light communication systems. As the wavelength of light used in such communication became more than 1.0 .mu.m for reducing the transmission loss in the fiber, it was found that silicon was unable to respond adequately thereto, whereas germanium responds well to light having a wavelength in range of 1.5 .mu.m. And thus, the utility of germanium in the field of light communication drew attention of the researchers in the art. However, it was generally believed that the established technique for manufacturing a silicon photodiode could not be applied to germanium because, whereas p-type layers can easily be formed into the silicon bulk by diffusion of boron, boron could not be used to diffuse deeply enough to form p-type layers in a germanium bulk.
It has been known that beryllium, when diffused into germanium at high temperature, forms a p-type layer. However, since its oxide is toxic, its utility as a p-type dopant was not considered in practice. For example, Diffusion in Semiconductors by B. L. Sharma, Trans Tech Publication of Clausthal-Zellerfeld, Germany reports, at P. 107, that solid solubility of beryllium in germanium is 4.times.10.sup.16 cm.sup.-3 at a maximum temperature of 920.degree. C. The same literature also presents at P. 89 that the diffusion coefficient of beryllium is 8.9.times.10.sup.-13 cm.sup.2 /sec., a value substantially equal to that of zinc or boron. This tended to divert the attention of the researchers in this art away from utilizing beryllium as a p-type diffusant for germanium. The general trend was to seek a suitable p=type diffusant in group III of the periodic table, and n-type dopant in group V. Although Zn is in group II, researchers neglected to look into elements in group II in their efforts to find a suitable p-type dopant to be diffused into germanium because of the lengthy heat treatment at high temperatures necessary with zinc.